The primary goal of this research proposal is to determine the mechanisms of conditioning that mediate the effects of exposure to the unconditioned stimulus (US) alone on both the acquisition and retention of a conditioned response (CR) established through pairings of a conditioned stimulus (CS) and US. This research program is guided by the view that Pavlovian conditioning paradigms serve as useful models for the study of behavioral adaptation of the organism to biologically significant events, and that the behavioral effects of exposure to the US alone are a consequence of engaging such adaptional processes. In this fashion, an analysis of these effects serves to delineate the nature of the mechanisms of conditioning, identify important features of the US and the unconditioned response (UR) that promote associative learning, and extend the set of circumstances to which the laws of conditioning apply. The specific goal of this proposal is to determine the mechanisms that mediate the effects of pre- and post-conditioning exposure to an electric shock US on conditioned suppression of responding in the rat. To the extent that the effects of pre- and post-conditioning exposure to shock are governed by similar mechanisms of conditioning, then implementing parallel operations within these two procedures should yield converging lines of evidence about the nature of these mechanisms. The present designs consider the possible roles of conditioning of situational stimuli as specified by the Rescorla-Wagner (1972) model, associatively generated priming as specified by Wagner (1979), and changes in the memorial representation of the US as specified by Rescorla (1974). Further, preliminary experiments examine possible physiological substrates of these processes.